1. Field of the Invention
This invention relates to hydrophilic plastic materials useful as soft contact lenses, and more particularly relates to treatment of soft hydrophilic contact lenses to make them more resistant to protein deposition and diffusion in use, and to improve their mechanical strength.
2. Background Information
It has long been known to fabricate contact lenses from rigid materials such as glass and clear plastics, e.g. essentially hydrophobic acrylic-type polymers such as polymethyl methacrylate and the like. Although more safely handled and used than glass, such hydrophobic plastic materials have been only moderately successful as contact lenses, being too hard and uncomfortable to the wearer. More recently, contact lens compositions have been developed from hydrophilic-type polymers which are softer and generally more easily accomodated by the eye. Thus, the use of hydrophilic polymer lens compositions is becoming of increasing importance in ophthalmological practice.
Hydrophilic polymers useful as soft contact lenses typically are lightly cross-linked copolymers derived from one or more hydroxyl group-containing monomers. In their hydrated state, these polymers are known generally in the art as "hydrogels", and in their dry state as "xerogels". A xerogel in the shape of a contact lens is referred to as a "replica". These gels can be defined as coherent, three-dimensional polymer structures or networks which have the ability to absorb or imbibe large quantities of water without dissolution therein. At the present time, the specific class of polymer hydrogels which have gained particular commercial acceptance as soft contact lenses are those derived from acrylic esters. U.S. Pat. Nos. 2,976,576 and 3,220,960, issued to Wichterle and Lim, are early patents which describe the use of acrylic ester hydrogels for the manufacture of soft contact lenses. Many subsequent patents, as well as technical articles, are directed to the preparation of numerous other acrylic ester-type hydrogels which differ primarily in the type and/or percentage of comonomers contained therein. Examples of such later patents include U.S. Pat. Nos. 3,361,858; 3,542,097; 3,647,736; 3,700,761; 3,728,315; 3,926,892; 4,045,547; 4,056,496; 4,095,877; 4,109,070; and 4,113,686.
In the main, acrylic ester hydrogels are all derived by copolymerizing a major amount of a water-soluble monoester of acrylic or methacrylic acid in which the ester moiety contains at least one hydrophilic group and a minor amount of a bifunctional diester of acrylic or methacrylic acid which crosslinks the hydrophilic group-containing monomer as it polymerizes.
Although presently used contact lenses fabricated from polymer hydrogels are much softer than the prior hard contact lenses and can be accomodated by the wearer with relatively little discomfort, they nevertheless have some disadvantageous properties and have not been completely satisfactory. Hydrogel lenses favor the growth of pathogenic bacteria and fungi on their surfaces and in their pores. If the lenses are not regularly cleaned and sterilized, or if they are stored in contaminated solutions, the pathogens can be easily sorbed by the lens material due to its flexible, hydrophilic polymer structure. Corneal-damaging chemical residues from improper cleaning and/or sterilization techniques may likewise be introduced into the lens material in this way. Also, because of this flexible, hydrophilic polymer structure, proteins and other normal substances in the eye environment can be easily deposited on and/or diffused through a lens with use. Further, these lenses are mechanically much weaker than hard lenses, and are subject to damage such as tearing, especially when hydrated.
U.S. Pat. No. 3,895,169 to Wichterle discloses a method of forming a hydrophilic layer on an acrylate resin which comprises partially hydrolyzing and re-esterifying the surface with a mixture of a strong acid and a polyhydroxy compound, e.g. sulfuric acid and ethylene glycol.
U.S. Pat. Nos. 4,097,657 and 4,128,318 to Sieglaff et al. disclose the treatment of soft contact lenses with a range of compounds including aliphatic dithiols and aliphatic amines (the '657 patent) and aliphatic aminoalcohols (the '318 patent). The lenses in these patents are apparently treated in the hydrogel (hydrated) rather than in the xerogel (unhydrated) state. The treatment is stated to increase the resistance of the lenses to clouding and discoloration.
U.S. Pat. No. 4,214,014 to Hofer et al. discloses a method for surface treating hard lenses or dry soft contact lenses by a gas plasma discharge, preferably in an oxygen atmosphere, which is stated to reduce the deposition of proteins, etc. on the lens.
U.S. Pat. No. 4,379,893 to O'Malley et al. discloses a method for surface treating soft contact lenses, especially those containing 2-hydroxyethyl methacrylate (HEMA), by reacting the lens with a methyl or ethyl ester of an alkylene dicarboxylic acid, such as diethyl succinate, which is stated to reduce the deposition of proteins, etc. on the lens. U.S. Defensive Publication No. T102,101 to Drake et al. discloses treating lenses with an aqueous solution of an alpha-amino acid, aliphatic dicarboxylic acid, or aromatic dicarboxylic or tricarboxylic acid, etc., to produce a modified, less hydrophilic structure.
British Pat. No. 1,401,233 to Ceskoslovenska Akademie Ved (Wichterle) discloses a method for surface treating HEMA articles, such as contact lenses, by reacting the article with a sulfonating or phosphorylating agent, which is stated to increase the hydrophilicity of the surface. Swelling of the material of the article is disclosed, as is the impregnation, prior to the surface treatment, of the article with a tertiary amine, such as pyridine.
It would be desirable to develop a process for the treatment of soft contact lenses, especially in their dry or xerogel state, which significantly inhibits protein and lipid deposition and diffusion in use and improves the mechanical strength of the lens, while still allowing for comfortable wear in the hydrogel state.
In a first aspect, the present invention provides a process for treating soft contact lenses to inhibit protein and lipid deposition thereon.
Another aspect of the present invention is to provide a process for treating soft contact lenses to improve the extraction of organic-type impurities therefrom.
Still another aspect of the present invention is to provide a process for improving the mechanical strength of a soft contact lens, while still remaining comfortable to wear when hydrated.
Yet another aspect of the invention is to provide a soft contact lens which is resistant to the deposition of protein and lipids.